1. Field of the invention:
This invention relates to a method for the production of semiconductor devices which minimizes undesirable phenomena such as non-uniform segregation of tellurium at the interface of a p-n junction or a heterojunction in semiconductors of Groups III to V of the periodic table which contains Te
2. Description of the prior art:
Liquid-phase epitaxy of semiconductors of Groups III to V of the periodic table is widely used as a conventional production method for semiconductor lasers and light-emitting diodes. When such devices are produced, Te is often used as an n-impurity. This makes it possible to obtain a crystal of high electron density with relative ease, and the process can be controlled readily.
However, there is a lamellar distribution of areas on the surface of the layer in which Te is used as an impurity where the density of Te is extremely high. In this case, when additional crystals are being grown continuously, undesirable effects often occur which result in certain characteristics exhibited by the semiconductor devices, such as a decrease in the uniformity of additional crystals grown on the surface of substrate crystals and an increase at the interface of recombination centers. For example, FIG. 3 shows a wafer which is produced as follows: On a p-GaAs substrate 31, a Te-doped AlGaAs current blocking layer (10.sup.19 cm.sup.-3) 32 having a thickness of 0.7 .mu.m and a non-Te-doped GaAs surface-protective layer 33 having a thickness of 0.1 .mu.m are grown by liquid phase epitaxy. When the thin semiconductor layer 33 is grown on the layer 32 containing Te in a high concentration, the morphology of the crystal surface is lamellar, as shown in FIG. 4. It has also been found that the non-Te-doped GaAs surface-protective layer 33 does not grow in the areas represented by the hatched areas in FIG. 4. The lamellar growth of this thin layer 33 originates in the non-uniform segregation of Te on the surface of the underlying GaAs current blocking layer 32. The form of the Te segregation affects the shape of the lamellae. Partial growth of the surface-protective layer 33 means that the AlGaAs current blocking layer 32 containing Al on the surface thereof is partially exposed to air. When this wafer is used as a substrate for subsequent crystal growth, growth is not possible in the exposed areas, becoming a major problem.
Problems also arise when this method is used in the production of semiconductor laser devices. FIG. 5 is a sectional view of a common semiconductor laser device of double heterostructure. On an n-GaAs substrate 51, a Te-doped n-Al.sub.x Ga.sub.1-x As cladding layer 52, a Mg-doped p-Al.sub.y Ga.sub.1-y As active layer 53, a Mg-doped p-Al.sub.x Ga.sub.1-x As cladding layer 54 and a Zn-doped p.sup.+ -GaAs ohmic contact layer 55 are successively grown by liquid phase epitaxy. This device has the same structure as the above-mentioned one in that the active layer (the p-Al.sub.y Ga.sub.1-y As active layer) 53 is formed on the Te-doped crystal cladding layer (the n-Al.sub.x Ga.sub.1-x As cladding layer) 52. The amount of Te doped here is 10.sup.18 cm.sup.-3, which is about one tenth of that of the example given above. For that reason, the undesirable effect of partial growth of the active layer 53 does not occur. However, various analyses suggest that the interface between the n-cladding layer 52 and the active layer 53 has scattered regions of mixed crystals and of non-radiate recombination centers. These have undesirable effects on the characteristics of the finished semiconductor laser device. Some examples of such undesirable effects include an increase in threshold current, a lowering of the differential quantum efficiency, and a decrease in the life-span of the device.